38. Count and Say

1. Questions

The count-and-say sequence is a sequence of digit strings defined by the recursive formula:

• countAndSay(1) = "1"
• countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is then converted into a different digit string.

To determine how you "say" a digit string, split it into the minimal number of groups so that each group is a contiguous section all of the same character. Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.

For example, the saying and conversion for digit string "3322251":

Given a positive integer n, return the n^th term of the count-and-say sequence.

2. Examples

Example 1:

Input: n = 1
Output: "1"
Explanation: This is the base case.

Example 2:

Input: n = 4
Output: "1211"
Explanation:
countAndSay(1) = "1"
countAndSay(2) = say "1" = one 1 = "11"
countAndSay(3) = say "11" = two 1's = "21"
countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"

3. Constraints

• 1 <= n <= 30

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/count-and-say 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

主要采用双指针法。

right的索引刚好是str下一个循环的开始。

class Solution {
  public String countAndSay(int n) {
    String str = "1";
    for (int i = 1; i < n; i++) {
      StringBuffer sb = new StringBuffer();
      int left = 0;
      int right = 0;
      while(right < str.length()) {
        while (right < str.length() && str.charAt(right) == str.charAt(left)) {
          right++;
        }
        sb.append(right - left).append(str.charAt(left));
        left = right;
      }
      str = sb.toString();
    }
    return str;
  }
}